1. Technical Field
The present invention relates to folding bicycles and, more particularly, to a folding bicycle frame with enhanced strength.
2. Description of Related Art
A conventional bicycle frame, as illustrated in FIG. 1, primarily comprises a seat tube 1 connected with a top tube 2, a down tube 3, a pair of seat stays 4 and a pair of chain stays 5 connected, respectively. Each of the top tube 2 and the down tube 3 has a front end thereof combined with a head tube 6, respectively. Each of the seat stays 4 and the chain stays 5 has a rear end thereof jointed at a rear fork tip. It can be observed that the seat tube 1, the top tube 2, and the down tube 3 collectively construct a triangular frame as a main force structure of the entire bicycle frame. The integrity of the force structure ensures the structural strength and resistance to compression, bend and deformation. However, a conventional folding bicycle frame typically has folding joints 7, 8 arranged at the middle parts of the top tube 2 and the down tube 3. Such design unquestionably affects the integrity of the aforementioned triangular frame and results in weakening the structural strength and resistance to compression, bend and deformation of the bicycle frame.
U.S. Pat. No. 4,900,047 (herein after referred to as '047 Patent) discloses a folding bicycle of another type. FIG. 1 of '047 Patent shows a seat tube with a top thereof connected to a top tube and a pair of seat stays. The seat tube further has a bottom thereof connected with a down tube, and a pair of chain stays. In a lateral perspective, the top tube, the down tube, the chain stays and the seat stays collectively construct a quadrilateral frame. The seat tube is positioned across the quadrilateral frame as a diagonal. Actually, the seat tube is composed of a first seat tube and a second seat tube are arranged in a partially concentric telescoping manner. The first seat tube is connected with the top tube and the down tube, while the second seat tube is connected with the set stays and chain stays. By the relative rotation between the first and second seat tubes, the quadrilateral frame can be folded or expanded. After the quadrilateral frame is expanded, a pair of quick release binders may be implemented to fix the frame at the expanded position.
However, the above design may have some disadvantages. First, since the seat tube component is constructed by concentrically placing the first seat tube inside the second seat tube, as shown in FIGS. 2A through 2D of '047 Patent, the joining area between the two seat tubes is not large enough to provide sufficient joining strength. Such insufficient joining strength appears on the main force structure of the bicycle frame may liable to render slant and deformation of the bicycle frame.
As '047 Patent implements the concentric rotation between the first and second seat tubes to fold or expand the quadrilateral frame, the front wheel of the bicycle comes close to and stay abreast the rear wheel of the bicycle when the bicycle is folded. However, since the front frame member and the rear frame member still include a relatively larger angle, the folded bicycle still consumes a relatively larger floor space for storage.